Ethanol (alcohol) is widely used in industries not only for beverages as alcoholic drinks but also for chemical industry, food industry and pharmaceutical products. Ethanol for alcoholic drinks and food additives is called as a brewed alcohol or a fermented alcohol and produced from natural raw materials such as molasses, sweet potatoes, and cereals by fermentation.
Industrial alcohols other than brewed alcohols are produced from ethylene as a raw material by chemical synthesis, thus called as synthetic alcohols. Almost all of ethanol other than the brewed alcohol and pharmaceutical alcohol is called as a denatured alcohol that contains an added substance such as methanol or isopropyl alcohol. A denatured alcohol used for external preparations or cosmetics is made to be unsuitable for drink by adding isopropyl alcohol which is less toxic than methanol or by giving bitterness or odor, although it does not contain methanol as a denaturant.
Recently, in Japan, a brewed alcohol is produced by a method for producing highly pure ethanol containing negligible impurities by purifying an imported alcohol with a highly advanced Japanese distillation mode, which imported alcohol is obtained overseas by distilling fermented mash from a natural raw material using a relatively simple distiller (referred to a “crude alcohol”). As an example of the highly advanced distillation mode, a distiller of super allospase mode is used (Non-Patent Document 1). In this mode, most of impurities such as low-boiling impurities such as aldehyde and methanol, and middle- and high-boiling impurities containing fusel oil components such as 1-propanol are separated using a number of columns such as a fermented mash column (A column), a separation column (A1 column), a concentration column (A2 column), an extraction column (D column), a rectifying column (B column), a purification column (C column), an impurities treatment column (G column), and a reduced pressure column (H column). Particularly, in the D column, hot water is injected from the top of the column to keep the ethanol concentration within the column at approximately 10 w/w %, thereby changing the relative volatility between ethanol and the other low-boiling and middle- to high-boiling impurities to effectively separate those impurities at the top of the column. However, it is known that the crude alcohol contains ethanol with a concentration of 90% by mass or more, but also contains a trace of other impurities that cannot be separated by the super allospase mode.
In the category of ethanol (alcohol), there is “absolute ethanol” (absolute alcohol) that contains 99.5 v/v % or more of ethanol at 15° C. General methods for producing an absolute alcohol include azeotropic distillation. That is, hydrous ethanol is azeotropically distilled with ethyl acetate or benzene to attain an ethanol content of 99.5 v/v %. It is difficult, however, to avoid mixing of a trace of ethyl acetate or benzene in the absolute alcohol obtained by azeotropic distillation. Accordingly, it is not preferable in some cases to adopt absolute ethanol produced by azeotropic distillation as a raw material for a substance that directly contacts with a human body such as an external preparation or a cosmetic.
As an alternative technique to the azeotropic distillation, a technique for producing an absolute alcohol by membrane separation has been developed. For example, a method for producing an absolute alcohol by combining distillation and membrane separation is known (for example, Patent Documents 1 and 2).
Patent Document 1 proposes a separation apparatus including a distillation means and a membrane separation means having a zeolite membrane for separating mixed vapor which distills off from the column top of the distillation means. This document describes converting an ethanol/water liquid mixture to vapor having an ethanol concentration of 91.0% by mass (equivalent to about 94 v/v %) by the distillation means, and then purifying the vapor till the ethanol concentration reaches 99.5% by mass (equivalent to 99.69 v/v %) through the zeolite membrane.
Patent Document 2 also discloses a separation apparatus of almost the same concept as in Patent Document 1. This apparatus improves the separation performance by being equipped with a plurality of separation membrane modules.
Further, a purification treatment method for obtaining an absolute alcohol from a fermented aqueous alcohol solution by a membrane separation technique is known (Patent Documents 3-5). It is considered that according to this technique, 99.8% by mass (equivalent to 99.88 v/v %) of absolute ethanol can be obtained with good energy efficiency by the purification treatment of a fermented aqueous ethanol solution with an ethanol concentration of 7.3% by mass (equivalent to about 9 v/v %). In this technique, however, only separation of impurities through two columns of fermented mash column and distillation column is performed because importance is given to energy efficiency, and the ethanol concentration in the distillation column is at most 85-90% by mass (equivalent to about 90-93.5 v/v %).
Thus, there is no problem such as mixing of a trace of ethyl acetate and benzene as in azeotropic distillation in the combination of distillation and membrane separation as described in Patent Documents 1-5. In addition, an absolute alcohol obtained by the combination of distillation and membrane separation is being put to practical use as bioethanol fuel to be used instead of petroleum.